Water-redispersible powders of film-forming polymers having a core/shell structure

ABSTRACT

The present invention relates to a water-redispersible powder based on film-forming polymers which are substantially water-insoluble and which are prepared by aqueous emulsion polymerization of monomers containing ethylenic unsaturation and with a “core/shell” structure comprising a hydrophobic core based on a polymer exhibiting a Tg (glass transition temperature) of between −30 and 25° C., preferably between −10 and 20° C., and a shell, the shell being bonded to the core by covalent bonds resulting from the reaction of an agent for grafting the shell onto the ethylenic unsaturations remaining in the core, characterized in that the shell is prepared by emulsion polymerization, on the core particles, of a mixture of monomers comprising at least one monomer chosen from styrene and a C 1 -C 12  alkyl acrylate or methacrylate, at least one monomer carrying a saccharide group and exhibiting at least one ethylenic unsaturation, and at least one grafting agent.

This application is an application under 35 U.S.C. Section 371 ofInternational Application Number PCT/FR96/00815, filed on May 31, 1996.

The present invention relates to water-redispersible powders based onfilm-forming polymers with a “core/shell” structure, the saidfilm-forming polymers being prepared by aqueous emulsion polymerizationof monomers containing ethylenic unsaturation.

Redispersible powders of the above type, which are generally obtained byspray-drying polymer latices, are already known. These powders have agreat commercial advantage because they can be marketed as they are tothe various users who redisperse them in water to prepare formulationswhich are suitable for the conventional uses of latices, namelyespecially the preparation of paper-coating compositions, paintcompositions, adhesive coatings (pressure-sensitive adhesives, tilingadhesives) and additives to hydraulic binders of the mortar or concretetype.

It is understood that the sale of such powders which can be redispersedby the user into a pseudo- or neolatex has, when compared with the saleof the initial latex, the immense advantage of not comprising water,which generally constitutes at least 50% of the weight of the latex, andthis entails considerable savings in transport and handling.

However, such powders must exhibit the following properties to bemarketable:

they must be stable in storage,

they must be easily redispersible in water at ambient temperature andmust produce a neolatex of narrow particle size which is suitable forthe targeted application,

for the application as additive to hydraulic binders, more particularlytargeted in the case of the present invention, the additive resultingfrom the redispersible powder must be compatible with the hydraulicbinder, must allow the hydraulic binder to set solid without delayingthe setting process and to impart to the coatings comprising the bindercontaining the additive a correct adhesiveness to the usual substrates,even in the moist state, as well as good mechanical properties,

they must be redispersible in a wide pH range,

they must exhibit a good redispersion in a medium containingmultivalent, in particular alkaline, ions.

The objective of the present invention is to propose a redispersiblepowder exhibiting all of the desired properties indicated above, and aprocess for the preparation of this powder.

To this end, the invention relates first of all to a water-redispersiblepowder based on film-forming polymers which are substantiallywater-insoluble and which are prepared by aqueous emulsionpolymerization of monomers containing ethylenic unsaturation and with a“core/shell” structure comprising:

a hydrophobic core based on a polymer exhibiting a Tg (glass transitiontemperature) of between −30 and 25° C., preferably between −10 and 20°C., and

a shell, the shell being bonded to the core by covalent bonds resultingfrom the reaction of an agent for grafting the shell onto the ethylenicunsaturations remaining in the core,

characterized in that the shell is prepared by emulsion polymerization,on the core, of a mixture of monomers comprising:

at most 90%, preferably from 50 to 80% by weight of at least one monomerchosen from styrene and a C₁-C₁₂ alkyl acrylate or methacrylate,

at most 50%, preferably from 15 to 40% by weight of at least one monomercarrying a saccharide group and exhibiting at least one ethylenicunsaturation, and

at most 10%, preferably at most 5% by weight of at least one graftingagent chosen from the (meth)allyl esters of (meth)acrylic acid,(meth)allyl esters of the mono- and diesters of maleic, fumaric anditaconic acids, and alkene derivatives of amides of acrylic andmethacrylic acids, such as N-methallylmaleimide.

The invention also relates to a pseudolatex obtained by redispersion inwater of a redispersible powder according to the invention.

The invention also relates to a process for the preparation of a powderaccording to the invention, which consists in carrying out the followingstages:

the core is prepared by aqueous emulsion polymerization, then

a mixture of monomers is polymerized in aqueous emulsion on the core inthe presence of a grafting agent and of an organophilic polymerizationinitiator, whereby the shell is formed, then

water is removed from the reaction mixture.

Finally, the invention relates to the use of the said pseudolatices andof the redispersible powders according to the invention as additives tohydraulic binders, adhesives, paper-coating compositions and paints.

The invention relates first of all to a water-redispersible powder basedon film-forming polymers which are substantially water-insoluble andwhich are prepared by aqueous emulsion polymerization of monomerscontaining ethylenic unsaturation and with a “core/shell” structurecomprising:

a hydrophobic core based on a polymer exhibiting a Tg (glass transitiontemperature) of between −30 and 25° C., preferably between −10 and 20°C., and

a shell, the shell being bonded to the core by covalent bonds resultingfrom the reaction of an agent for grafting the shell onto the ethylenicunsaturations remaining in the core,

characterized in that the shell is prepared by emulsion polymerization,on the core particles, of a mixture of monomers comprising:

at most 90%, preferably from 50 to 80% by weight of at least one monomerchosen from styrene and a C₁-C₁₂ alkyl acrylate or methacrylate,

at most 50%, preferably from 15 to 40% by weight of at least one monomercarrying a saccharide group and exhibiting at least one ethylenicunsaturation, and

at most 10%, preferably at most 5% by weight of at least one graftingagent chosen from the (meth)allyl esters of (meth)acrylic acid,(meth)allyl esters of the mono- and diesters of maleic, fumaric anditaconic acids, and alkene derivatives of amides of acrylic andmethacrylic acids, such as N-methallylmaleimide.

The powder according to the invention is composed of particles based onwater-insoluble film-forming polymers exhibiting a core/shell structure.These core/shell particles consist of a core formed by a hydrophobic andsoft polymer, that is to say exhibit a Tg of between −30 and +25° C.,preferably between −10 and +20° C., which comprises remaining ethylenicunsaturations, and of a shell based on a hard, hydrophilic polymerexhibiting a Tg higher than 55° C., preferably higher than 90° C. Theshell is bonded to the core by covalent bonds resulting from thereaction of an agent for grafting the shell onto the ethylenicunsaturations remaining in the core.

The glass transition temperature is determined by the differentialthermal analysis method.

Such particles of core/shell polymers which are substantially insolublein water are prepared from a polymer latex by a two-stage polymerizationprocess such as, for example, that described in patent U.S. Pat. No.4,876,313, cited as reference, according to which, during a first stage,the core formed by a hydrophobic and soft polymer is prepared and,during a second stage, the shell formed by a hard and hydrophilicpolymer is grafted onto the core.

In general the core is prepared by polymerization of a mixture ofmonomers comprising from 99.9 to 92% of at least one monomer chosen fromstyrene, butadiene and C₁-C₁₂ alkyl acrylates and methacrylates and from0.1 to 8% of at least one monomer chosen from a carboxylic acidcontaining ethylenic unsaturation, an ester of unsaturated carboxylicacid containing a hydroxyalkyl functional group or a monomer containingan amide functional group. The core is preferably prepared bypolymerization of a mixture of monomers based on styrene and butadiene.

The more or less hydrophobic nature of the core can be adjusted with thecarboxylic acid monomers containing ethylenic unsaturation, such asacrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaricacid, or a monomer ester of unsaturated carboxylic acid containing ahydroxalkyl functional group, like hydroxyethyl acrylate andhydroxypropyl acrylate or a monomer containing an amide functional groupchosen, for example, from methylolacrylamide, acrylamide, methacrylamideand methylmethacrylamide.

A person skilled in the art knows how to choose the respectivequantities of “hard” monomers (for example styrene and methylmethacrylate) and of “soft” monomers (for example butadiene and butylacrylate) with a view to obtaining a polymer which has the desired Tg ofbetween −30 and +25° C.

The monomers used during this first stage may be introduced as a mixtureor separately and simultaneously into the reaction mixture, eitherbefore the beginning of the polymerization, once only, or during thepolymerization, using successive fractions or continuously.

The polymerization of the monomers forming the core is carried out inthe presence of an emulsifier and of a polymerization initiator.

The emulsifying agent employed is generally the conventional anionicagents represented especially by the alkali metal salts of fatty acids,alkyl sulphates, alkylsulphonates, alkylaryl sulphates,alkylarylsulphonates, aryl sulphates, arylsulphonates, sulphosuccinatesand alkyl phosphates. They are employed in a proportion of 0.01 to 5% byweight relative to the total weight of the monomers.

The initiator for emulsion polymerization, which is water-soluble, isrepresented more particularly by hydroperoxides such as aqueous hydrogenperoxide, tert-butyl hydroperoxide, and by persulphates such as sodiumpersulphate, potassium persulphate and ammonium persulphate. It isemployed in quantities between 0.05 and 2% by weight relative to thetotal of the monomers. These initiators are optionally used incombination with a reducing agent such as sodium bisulphite orformaldehyde sulphoxylate, polyethylene amines, sugars: dextrose,sucrose, or metal salts. The quantities of reducing agent which areemployed vary from 0 to 3% relative to the total weight of the monomers.

The reaction temperature, which is a function of the initiator used, isgenerally between 0 and 100° C. and preferably between 50 and 80° C.

A transfer agent may be employed in proportions ranging from 0 to 3% byweight relative to the monomer(s), and is generally chosen frommercaptans such as n-dodecyl mercaptan, tert-dodecyl mercaptan,cyclohexene and halogenated hydrocarbons such as chloroform, bromoformand carbon tetrachloride.

At the end of the first stage of preparation of the core, the secondstage of preparation of the shell is begun by adding directly to thelatex comprising the core particles a mixture of monomers which issuitable for producing a hard and hydrophilic shell as defined above.

Furthermore, during the second stage an effective quantity of a graftingagent is added during the emulsion polymerization in order to bond theshell by covalent bonds to the ethylenic unsaturations remaining in thecore, so as to graft the shell firmly onto the core.

Up to 90% by weight of the base monomers of the shell are chosen fromstyrene and a C₁-C₁₂ alkyl acrylate or methacrylate. The alkyl acrylateor methacrylate may be chosen, for example, from methyl or ethyl(meth)acrylate, butyl (meth)acrylate and 2-ethylhexyl (meth)acrylate.

The hydrophilic nature of the shell is adjusted by adding up to 50% byweight of a monomer carrying a saccharide group and exhibiting at leastone ethylenic unsaturation. This monomer carrying a saccharide group andexhibiting at least one ethylenic unsaturation preferably carries asaccharide group chosen from mono- and disaccharides. In the case ofmonosaccharide groups, these may be, for example, glucose, galactose orfructose. In the case of disaccharide groups, these may be, for example,sucrose, lactose or maltose.

Very advantageously, the saccharide group is sucrose. In this lattercase the monomer carrying a sucrose group and exhibiting at least oneethylenic unsaturation may be chosen from O-methacryloylsucrose,O-vinylbenzylsucrose or O-acetalstyrylsucrose and their alkylated and/oracetylated derivatives in which the alkyl groups contain a number ofcarbons of between 1 and 4.

The important point is that the shell must have a substantially higherhydrophilic nature than the core. According to an embodiment of theinvention the shell may be insoluble in an alkaline medium.

A person skilled in the art knows how to choose the respectivequantities of “hard” monomers and of “soft” monomers with a view toobtaining a polymer which has the desired Tg of the shell higher than55° C.

The polymerization mixture additionally contains a grafting agent insufficient quantity and which is sufficiently reactive to producecovalent bonds between the shell and ethylenic bonds present in the corein order to graft the shell firmly onto the core.

This grafting agent consists of an organic compound containing at leasttwo ethylenic unsaturations exhibiting different reactivities duringemulsion polymerization reactions, the more reactive unsaturationreacting with the monomers with the result that the grafting agent formsan integral part of the polymer constituting the shell, the otherunsaturation being insufficiently reactive to undergo polyaddition withthe ethylenic unsaturations of the other monomers, but beingsufficiently reactive to react with the remaining double bonds of thecore to bind the shell to the core by covalent bonds.

An effective grafting agent which may be mentioned is the alkene andmore particularly (meth)allyl esters of (meth)acrylic acid, such asallyl methacrylate, allyl acrylate, (meth)allyl esters of the mono- anddiesters of maleic, fumaric and itaconic acids, and alkene derivativesof amides of acrylic and methacrylic acids, such asN-methallylmaleimide.

The grafting agent may be added in a quantity which is generally at most10%, preferably at least 0.2%, still more preferably of between 0.5 and5% or even 0.5 and 3% by weight of the shell monomers.

According to a preferred embodiment of the invention the shell isprepared by emulsion polymerization of a mixture of methyl methacrylate,of O-methacryloylsucrose and of a grafting agent.

According to another preferred embodiment of the invention the shell isprepared by emulsion polymerization of a mixture of styrene, ofO-vinylbenzylsucrose and of a grafting agent.

The emulsion polymerization of the shell is performed in conditionssimilar to those of the core.

In general, in the case of particles of redispersible powder accordingto the invention, the shell constitutes from 50 to 5%, preferably from30 to 10%, by weight of the total core+shell weight.

The redispersible powder according to the invention may additionallyinclude at least one inorganic filler. The latter may exhibit a variableparticle size. It is recommended to employ as an inorganic filler afiller chosen especially from calcium carbonate, kaolin, bariumsulphate, titanium oxide, talc, alumina hydrate, bentonite, silica andcalcium sulphoaluminate (satin white).

The powder may include from 0.5 to 40, preferably from 2 to 20, parts byweight of the said filler per 100 parts by weight of film-formingpolymers.

The particle size of the powder is generally smaller than 500 μm,preferably smaller than 100 μm.

The powders according to the invention generally exhibit a pH of between4 and 9. They can be redispersed in any medium of pH of between 1 and13, preferably between 4 and 10.

In most cases the powders according to the invention are completelyredispersible in water at ambient temperature, merely by agitation.Completely redispersible is intended to mean a powder in accordance withthe invention which, after the addition of a suitable quantity of water,makes it possible to obtain a pseudolatex whose particle size of theparticles is substantially identical with the particle size of the latexparticles present in the starting emulsion.

The powders according to the invention are stable in storage; they canbe easily redispersed in water in the form of pseudolatex and employeddirectly in the form of powder or in the form of pseudolatex in all theknown fields of application of latices.

The invention also relates to a pseudolatex obtained by redispersion inwater of a redispersible powder as defined above.

The invention also relates to the process for the preparation of theredispersible powders according to the invention, which consists inimplementing the following stages:

the core is prepared by aqueous emulsion polymerization, then

a mixture of monomers is polymerized in aqueous emulsion on the core inthe presence of a grafting agent and of an organophilic polymerizationinitiator, whereby the shell is formed, then

water is removed from the reaction mixture.

The stages described above are therefore implemented so as to form alatex of film-forming polymers with a core/shell structure and duringthe second stage of polymerization of the shell, as described above, thesaid polymerization is carried out in the presence of a grafting agentand of an organophilic polymerization initiator.

The organophilic polymerization initiator exhibits a high affinity forthe particles of polymer constituting the core so as to improve thegrafting. It is generally a peroxide or a perester such as especiallycumene hydroperoxide, dicumyl peroxide, tert-butyl perneodecanote,tert-butyl per-3,5,5-trimethylhexanoate, bis(3,5,5-trimethylhexanoyl)peroxide, dioctanoyl peroxide, dilauroyl peroxide and dibenzoylperoxide. It is more particularly recommended to employ the tert-butylperbenzoate/ascorbic acid combination. The weight content of thiscombination may be between 0.5 and 5% by weight relative to the totalweight of the monomers of the shell.

It should be noted that if a water-soluble initiator, such as ammoniumpersulphate, is employed instead of the organophilic initiator, a powderof mediocre redispersibility is obtained.

It is desirable to employ a transfer agent to adjust the molecularmasses of the shell polymer. The same transfer agents may be employed asthose mentioned above for preparing the core, as well as alkyl (C₁-C₁₂)mercaptan alkanoates (C₁-C₆), such as methylmercapto propionate.

The latex obtained generally exhibits a particle size of between 0.1 and0.3 μm.

An inorganic filler as defined above may be optionally added to thelatex with a view to making it easier to prepare the pulverulentcomposition. The respective contents of the various constituents arechosen such that the dried pulverulent compositions have the compositiondefined previously.

The water of this emulsion is next removed and the product obtainedpulverized to obtain a powder. The stages of removal of the water fromthe latex emulsion and of obtaining a powder may be separate orcombined. It is thus possible to employ a process of freezing, followedby a stage of sublimation or of freeze-drying, of drying or of drying byatomizing (spray-drying).

The water is preferably removed after the pH of the reaction mixture hasbeen adjusted between 6 and 9.

Drying by atomizing is the preferred process because it makes itpossible to obtain directly the powder with the desired particle sizewithout necessarily going through the grinding stage. The drying byatomizing can be performed in the usual manner in any known apparatussuch as, for example, an atomizing tower combining a pulverizationproduced by a nozzle or a turbine with a stream of hot gas.

The entry temperature of the hot gas (generally air) at the head of thecolumn is preferably between 100 and 115° C. and the exit temperature ispreferably between 55 and 65° C.

Inorganic fillers may be added to the aqueous emulsion of startingpolymer or else directly into the final pulverulent composition.

All or part of the inorganic filler is preferably introduced during thepulverization stage in the process of drying by atomizing(spray-drying).

The presence of these inorganic fillers in the emulsion promotes thepreparation of the powder and its stability in storage, avoidingaggregation of the powder, that is to say its caking.

The redispersible powders or the pseudolatices according to theinvention are very particularly suitable in the building industry asadditives to the mixtures of hydraulic mineral binders for theproduction of protective and decorative coatings and of adhesive mortarsand adhesive cements intended for the laying of tiling and of floorcoverings, where they are employed in a weight proportion of 1 to 10%,preferably of 2 to 5%. They are found to be particularly well suited tothe preparation of ready-for-use powdered products based on cement andon plaster. They do not increase the setting time of these binders anddo not exhibit or induce any caking in storage.

The powders of the invention or the pseudolatices derived therefrom arefurthermore usable in all the other fields of application of latices,more particularly in the field of adhesives, of paper coating and ofpaints. The powders according to the invention may additionally comprisethe usual additives, in particular biocides, microbiostats,bacteriostats and silicone and organic foamsuppressors.

The powders of the invention can also be employed as reactive powderswith polymers carrying isocyanate, epoxy or acidic bonds forcrosslinking the products for paint and coating.

In all that follows or that precedes, the parts and percentages areexpressed by weight, unless shown otherwise. Examples will now be given,no limitation being implied.

EXAMPLES Example 1

Overpolymerization of 4% by Weight of O-Methacryloylsucrose on a Seed ofStyrene-Carboxylated Butadiene Copolymer in the Presence of GraftingAgent.

1. 4 kg of a seed latex with a mass solids content of 49.5% are preparedby emulsion polymerization in the presence of ammonium persulphate of amixture of:

37% by weight of butadiene,

59% by weight of styrene, and

4% by weight of acrylic acid. tert-Butyl mercaptan is employed astransfer agent in a proportion of 0.8% by weight relative to the monomermixture.

2. The following three solutions are prepared:

Solution A: mixture of 217.5 g of methyl methacrylate (MMA) and 1.9 g ofallyl methacrylate (ALMA), which is a grafting agent.

Solution B: mixture of 79 g of O-methacryloylsucrose in 150 g ofdeionized water.

Solution C: mixture of 2.9 g of ascorbic acid and of 2 g of sodiumdodecylbenzenesulphonate, which is an emulsifier.

3. The seed latex prepared in stage 1 is charged into a polymerizationreactor equipped with a stirrer, a condenser and reactant feed tubes. Itis heated for 1 h with stirring so as to reach a temperature of 80±1° C.

During this temperature rise 7.4 g of tertbutyl perbenzoate (which formsthe polymerization initiator with the ascorbic acid) are mixed with 26.3g of solution A and this mixture is introduced into the reactor duringits rise in temperature, at the time when the latter reaches 30±1° C.Then, when the temperature reaches 35±1° C., 27.3 g of solution B areadded to the reactor.

Finally, when the temperature of the reactants reaches 80±1° C. and hasstabilized, the remainder of solutions A and B is added to the reactorover a period of 3 h, and solution C over a period of 3 h 30 min.

At the end of the introduction of the reactants of solutions A, B and Cthe reactor mixture is maintained at 80±1° C. for 1 h.

The latex is then cooled and discharged from the reactor. It is foundthat no grain and no crust has formed on the wall of the reactor duringthe polymerization.

4. The latex obtained exhibits the following characteristics:

solids content: 51.1% by weight,

pH=5.0,

viscosity measured in Brookfield RTV-DV 11 at 50 rev/min: 180 mPa s,

particle size measured by Brookhaven

DCP 1000 photosedimentometer: 0.17±0.01 μm.

Example 2

Overpolymerization of 6% by Weight of O-Methacryloylsucrose on a Seed ofStyrene-Carboxylated Butadiene Copolymer in the Presence of GraftingAgent.

The operations carried out in Example 1 are repeated, except thatsolution B includes a mass of 118.8 g of O-methacryloylsucrose.

The latex obtained exhibits the following characteristics:

solids content: 51% by weight,

pH=5.0,

viscosity measured in Brookfield RTV-DV 11 at 50 rev/min: 320 mPa s,

particle size measured by Brookhaven

DCP 1000 photosedimentometer: 0.17±0.01 μm.

Example 3

Drying of the Latices Obtained in the Preceding Examples andRedispersion of the Powders Obtained.

The latices prepared from Examples 1 and 2 are converted into powder byspray-drying after adjustment of their pH to 7 with a 10% sodiumhydroxide solution. This drying is carried out in a spraying tower inwhich the hot air entry temperature is 105° C. and the exit temperature60° C. During the spraying operation an anticaking agent (kaolin) isadded to the latex in a proportion of 12% by weight.

The powders obtained exhibit the following characteristics:

excellent fluidity,

great stability in storage: no caking appears after storage for 2 weeksat 55° C.,

excellent wetting by water,

spontaneous, rapid and complete redispersion both in deionized water andin a concentrated (1M) CaCl₂ solution.

The particle size distributions of the pseudolatices obtained byredispersion of these powders in water, determined with a Brookhaven DCP1000 photosedimentometer, are identical with those of the latices ofExamples 1 and 2.

The powders exhibit good properties when they are employed in aproportion of 5% by weight in standard mortars. In particular, theyimprove their setting time and their mechanical properties.

Comparative Example 4

Overpolymerization of 4% by Weight of O-Methacryloylsucrose on aStyrene/Carboxylated Butadiene Seed Without Grafting Agent.

The procedure of Example 1 is implemented, leaving out the graftingagent, that is to say allyl methacrylate from solution A. The latexobtained exhibits the following characteristics:

solids content: 51.1% by weight,

pH=5.0,

viscosity measured in Brookfield RTV-DV 11 at 50 rev/min: 520 mPa s,

particle size measured by Brookhaven

DCP 1000 photosedimentometer: 0.16±0.01 μm.

After atomization by spray-drying according to the procedure of Example3, the powder obtained exhibits only a partial redispersibility: thepercentage of particles of size greater than 43 μm is 40%.

What is claimed is:
 1. A water-redispersible powder comprisingfilm-forming polymers which are water-insoluble and with a “core/shell”structure, the core and the shell being prepared by aqueous emulsionpolymerization of monomers containing ethylenic unsaturation, the powdercomprising: a hydrophobic core consisting of a polymer exhibiting aglass transition temperature of between −30 and 25° C., and a shell, theshell being bonded to the core by covalent bonds resulting from thereaction of an agent for grafting the shell onto the ethylenicunsaturations remaining in the core, wherein the shell is prepared byemulsion polymerization, on the core, of a mixture of monomerscomprising: from 50 to 90% by weight of at least one monomer chosen fromstyrene and a C₁-C₁₂ alkyl acrylate or methacrylate, from 15 to 50% byweight of at least one monomer carrying a saccharide group andexhibiting at least one ethylenic unsaturation, and from 0.2 to 10% byweight of at least one grafting agent which is an alkene ester or analkene amide of acrylic or methacrylic acid.
 2. A powder according toclaim 1, wherein the shell is prepared by emulsion polymerization of amixture of styrene, O-vinylbenzylsucrose, and a grafting agent.
 3. Apowder according to claim 1, wherein the shell constitutes from 50 to 5%by weight of the total core and shell weight.
 4. A powder according toclaim 1, wherein said powder comprises at least one inorganic filler. 5.A process for the preparation of a powder according to claim 1,comprising the steps of: (1) preparing the core by aqueous emulsionpolymerization, then (2) polymerizing the mixture of monomers for thepreparation of the shell in aqueous emulsion on the core in the presenceof a grafting agent and of an organophilic polymerization initiator,whereby the shell is formed, and then (3) removing water from thereaction mixture.
 6. A process according to claim 5, wherein theorganophilic polymerization initiator is a peroxide or a perester.
 7. Aprocess according to claims 5, wherein in step (3) the water is removedby spray-drying.
 8. A powder according to claim 1, wherein the graftingagent is an allyl- or methallylester of acrylic or methacrylic acid, anallyl- or methallylester of a mono- or diester of maleic, fumaric oritaconic acid, or an alkene amide of acrylic or methacrylic acid.
 9. Apowder according to claim 1, wherein the core is prepared bypolymerization of a mixture of monomers comprising from 99.9 to 92% ofat least one monomer selected from the group consisting of styrene,butadiene, and C₁-C₁₂ alkyl acrylates and methacrylates and from 0.1 to8% of at least one monomer selected from the group consisting of acarboxylic acid containing ethylenic unsaturation, an ester ofunsaturated carboxylic acid containing a hydroxyalkyl functional group,and a monomer containing an amide functional group.
 10. A powderaccording to claim 9, wherein the core is prepared by polymerization ofa mixture of monomers comprising styrene and butadiene.
 11. A powderaccording to claim 1, wherein the monomer carrying a saccharide groupand exhibiting at least one ethylenic unsaturation carries a saccharidegroup which can be a mono- or a disaccharide.
 12. A powder according toclaim 11, wherein the saccharide group is sucrose.
 13. A powderaccording to claim 11, wherein the monomer carrying a saccharide groupand exhibiting at least one ethylenic unsaturation isO-methacryloylsucrose, O-vinylbenzylsucrose, or O-acetalstyrylsucrose.14. A powder according to claim 1, wherein the shell is prepared byemulsion polymerization of a mixture of methyl methacrylate,O-methacryloylsucrose, and a grafting agent.
 15. A powder according toclaim 1, wherein the shell is prepared by emulsion polymerization of amixture of styrene, O-vinylbenzylsucrose, and a grafting agent.
 16. Apowder according to claim 1, wherein the shell constitutes from 50 to 5%by weight of the total core and shell weight.
 17. A powder according toclaim 1, wherein said powder comprises at least one inorganic filler.18. A pseudolatex obtained by redispersion in water of a powder asdefined in claim
 1. 19. A method of using a pseudolatex as defined inclaim 18 as additive to hydraulic binders, comprising the step of addingto said hydraulic binders a proportion of 1 to 10% by weight of saidpseudolatex.
 20. A method of using a powder as defined in claim 1, asadditive to hydraulic binders comprising the step of adding to saidhydraulic binders a proportion of 1 to 10% by weight of said powder. 21.A water-redispersible powder comprising film-forming polymers which arewater-insoluble and which are prepared by aqueous emulsionpolymerization of monomers containing ethylenic unsaturation and with a“core/shell” structure, wherein the hydrophobic core consists of apolymer exhibiting a Tg of between −10 and 20° C. and the shell isprepared by emulsion polymerization, on the core, of a mixture ofmonomers comprising: from 50 to 80% by weight of at least one monomerchosen from styrene and a C₁-C₁₂ alkyl acrylate or methacrylate, from 15to 40% by weight of at least one monomer carrying a saccharide group andexhibiting at least one ethylenic unsaturation, and at most 5% by weightof at least one grafting agent which is an alkene ester or an alkeneamide of acrylic or methacrylic acid.
 22. A powder according to claim21, wherein the grafting agent is an allyl- or methallylester of acrylicor methacrylic acid, an allyl- or methallylester of a mono- or diesterof maleic, fumaric or itaconic acid, or N-methallylmaleimide.
 23. Apowder according to claim 21, wherein the core is prepared bypolymerization of a mixture of monomers comprising from 99.9 to 92% ofat least one monomer selected from the group consisting of styrene,butadiene, and C₁-C₁₂ alkyl acrylates and methacrylates and from 0.1 to8% of at least one monomer selected from the group consisting of acarboxylic acid containing ethylenic unsaturation, an ester ofunsaturated carboxylic acid containing a hydroxyalkyl functional group,and a monomer containing an amide functional group.
 24. A powderaccording to claim 23, wherein the core is prepared by polymerization ofa mixture of monomers based on styrene and butadiene.
 25. A powderaccording to claim 21, wherein the monomer carrying a saccharide groupand exhibiting at least one ethylenic unsaturation carries a saccharidegroup which can be a mono- or a disaccharide.
 26. A powder according toclaim 25, wherein the saccharide group is sucrose.
 27. A powderaccording to claim 25, wherein the monomer carrying a saccharide groupand exhibiting at least one ethylenic unsaturation isO-methacryloylsucrose, O-vinylbenzylsucrose, or O-acetalstyrylsucrose.28. A powder according to claim 21, wherein the shell is prepared byemulsion polymerization of a mixture of methyl methacrylate,O-methacryloylsucrose. and a grafting agent.
 29. A pseudolatex obtainedby redispersion in water of a powder as defined in claim
 20. 30. Amethod of using a pseudolatex as defined in claim 29 as additive tohydraulic binders, comprising the step of adding to said hydraulicbinders a proportion of 1 to 10% by weight of said pseudolatex.
 31. Amethod of using a powder as defined in claim 21, as additive tohydraulic binders comprising the step of adding to said hydraulicbinders a proportion of 1 to 10% by weight of said powder.
 32. Awater-redispersible powder according to claim 21, comprising from 0.2 to10% by weight of at least one grafting agent which is an alkene ester oran alkene amide of acrylic or methacrylic acid.